The present invention relates generally to musical instrument amplifiers and more particularly concerns a solid-state amplifier designed to achieve a desired sound characteristic associated with the traditional tube-type amplifier.
As popular music has evolved through the past 30 years, so has the equipment used to produce such music. Through the 1960's and 70's, electric guitarists used vacuum tube amplifiers which combined both a pre-amplifier to achieve a specific, often distorted, guitar sound and a power amplifier to amplify the guitar signal to a level capable of driving guitar speaker cabinets in a single package. The 1980's saw a dramatic change in this philosophy as an increasing number of musical instrument companies began offering pre-amplifiers and power amplifiers as separate packages, thereby often providing more versatility to the musician.
While tube-type amplifiers provided a sound considered desirable to most guitarists, the advent of solid-state technology in the field of instrument amplification boasted many advantages over traditional tube-type designs. Solid-state amplifiers were decidedly more reliable and required far less maintenance than vacuum tube amplifiers. This, coupled with the fact that vacuum tubes themselves are becoming a rarity in all other fields, thereby increasing their cost and making it more difficult to find replacements, has increased the popularity of solid-state amplifiers in recent years.
Even so, the solid-state instrument amplifier has always carried with it a single common criticism that it cannot achieve the rich, warm sound characteristics commonly associated with tube-type guitar amplifiers. Solid-state amplifiers are commonly referred to as being "harsh" or "shrill" sounding. This criticism has spawned many attempts by manufacturers to replicate the "tube sound" with solid-state designs. Nearly all of these designs have been based on the theory that tube type and solid state amplifiers sound distinctly different from one another because a vacuum tube reacts differently when driven into clipping than a transistor. U.S. Pat. No. 4,987,381 recently issued to Brent K. Butler discloses a power amplifier design based on this theory. This patent discloses a hybrid MOSFET/bipolar design which "generates a distorted output signal rich in even harmonics."
It is a myth, however, that a tube generates predominantly even-order harmonics when distorted while a transistor produces predominantly odd-order harmonics. While a vacuum tube may react differently when driven into soft clipping, the hard clipping typically used by guitar players to generate distortion will result in the tube generating equal amounts of even and odd-order harmonics.
The present invention is, therefore, not based on these principles, but instead based on the premise that the high output impedance of a tube-type amplifier results in the impedance response of the guitar speaker having a distinctly audible effect on it's frequency response. Solid-state amplifiers provide a very low output impedance, and therefore the impedance response characteristics of the speaker itself are not audible to the listener. Thus it can be demonstrated that it is not the vacuum tube itself which provides the desirable sound, but rather the interaction of the impedance response and frequency response of the loudspeaker when connected to the high impedance output of a tube-type amplifier. In audio, the relation of an amplifier's output impedance to that of the load connected to its output terminals determines what is referred to as its Damping Factor. The Damping Factor is specified by dividing the load impedance by the output impedance of the amplifier.
Variations in amplitude as small as 1/4dB can be perceived by the human ear if over a broad enough bandwidth of the audio spectrum. With high fidelity tube amplifiers, an output impedance of as little as 1.5 ohms is considered high, and it's effects on a speaker's impedance response become audibly apparent and influence the listener's perception of subjective characteristics, such as "warmth," "sterility" and the like. Some of the most desirable tube-type guitar amplifiers, on the other hand, provide an output impedance as high as 30-40 ohms, and their interaction with the impedance response of the guitar speaker is therefore considerably greater than that of the Hi Fi amplifier. The result is a steep low-frequency resonance peak at approximately 90 Hz and a gradually rising amplitude above 1 KHz. The sharp boost of the low-frequency peak below 100 Hz is not a major element in the end audible outcome, as the typical guitar speaker cabinet's efficiency below 100 Hz is very poor. The amplitude of the signal may be equally boosted from 20 Hz to 100 Hz with similar results. However, the equally sharp drop above 100 Hz is a key element in producing the desired effect.
Some manufacturers of solid-state amplifiers have also included user-adjustable bass and treble controls to further shape the tone of the signal being amplified. These controls are typically shelving-type and cannot possibly replicate the interaction which takes place between a tube type amplifier and a guitar loudspeaker.
It would be possible to mimic the sound of a tube-type guitar amplifier with a solid-state amplifier by simply adding resistance to the output stage to match the tube amp's output impedance. This would cause a tremendous loss of power, especially considering that the most desirable tube-type guitar amplifiers have tremendously high output impedance ratings. To compensate for this loss by simply increasing the power of the solid-state amplifier would not only prove more costly, but would also produce a far greater degree of unwanted heat.
In view of these existing limitations, it is an object of the present invention to provide the much sought after tube-like sound of a guitar amplifier in a solid-state circuit. A further object of the present invention is to provide the sound of a tube-type amplifier possessing a high output impedance, and therefore mimic or simulate an audible interaction with the impedance plot of the loudspeaker, without increasing the output impedance of the amplifier. It is a further desired object of the present invention to provide a tube-type amplifier sound without alteration of the harmonic information of the amplifier. Yet another object is to provide a means by which the amount of simulated interaction may be defined by the user. And it is an object of the present invention to provide an instrument amplifier system which incorporates a speaker impedance frequency response circuit at the amplifier's input which parallels the impedance response characteristics of the most commonly used guitar loudspeakers.